A 30 V, 90 W lamp is to be perated on a 120 V D.C. line. For proper glow, a resistor of ………….. ohm should be connected in series with the lamp.

Options

A. 40
B. 10
C. 20
D. 30

Answer

D

Explanation

Resistance of the lamp, `R_(0)=(V^(2))/(P)`
`=((30)^(2))/(90)=10 Omega`.
Current in the lamp, `I=30//10=3Omega`.
When lamp is operated on a 120 V,
then resistance `R'=(V)/(I)=(120)/(3)=40 Omega`.
`therefore` For proper glow, the resistance required to be put in a series will be
`R=R'-R_(0)=40-10=30 Omega`.